The utilization of closed loop control of internal combustion engines for reducing emissions and enhancing performance has gone through an evolutionary process since the 1970's with the replacement of carburetor based systems with single port (monotronic) fuel injection controlled by utilizing the signal from an unheated oxygen sensor to determine the engine's air/fuel ratio. This has evolved into multi-port fuel injection systems with heated oxygen sensors. Currently, such technology from the automotive industry is being applied to improve emission control in small engines for motorcycle and off-road applications. However, these sensors in their present state are cost prohibitive for a vast majority of global applications.
Two major classes of oxygen sensors have been developed and have competed for the automotive market since the onset of closed-loop control. Voltaic sensors rely on voltage generation due to a chemical potential across an ion conductor (stabilized zirconia) situated between the exhaust gas and a reference gas, typically air, in accordance with the Nernst equation, which is well known to those of ordinary skill. This type of sensor undergoes a step-wise change in voltage, transitioning across stoichiometry, due to an abrupt change in oxygen concentration at that point.
A second type of sensor known as a resistive sensor relies on a step-wise change in resistance of a semiconductor material (typically titania-based) as exhaust gases transition across the stoichiometric boundary. Both classes of sensors must be heated to become functional.
Zirconia sensors have held the majority of market share, and as such have gone through the greatest evolutionary change. Initially, zirconia sensors were unheated and relied on the heat from exhaust gasses to bring them to a temperature at which they become functional. Heater elements were later added to hasten sensor activation (light-off time), and increase the numbers of possible mounting locations along the exhaust stream. Further improvements have included the use of an integrated heater with multi-layer packaging technology, i.e., a planar sensor.
More recently universal “wide-band” or “air/fuel” sensors have been developed providing the ability to determine the air-fuel ratio away from stoichiometry in a somewhat linear current vs. air-fuel relationship, as compared to the step change in voltage at stoichiometry in earlier types of sensors. Unfortunately, these sensors are very expensive, have complicated circuitry, and the size reduction potential is limited due to the need to have enough charge carriers to generate a signal. As such, they are therefore not suitable for the small engine market. By “small engine” is meant as defined by the Environmental Protection Agency, “ . . . those products rated less than or equal to 19 kilowatt (kW) (roughly equivalent to 25 horsepower [hp])” (Ref: Control of Emissions from Marine SI and Small SI Engines, Vessels, and Equipment—Final Regulatory Impact Analysis. EPA420-R-08-014, September 2008). This applies to single or multiple cylinder spark ignition or compression ignition engines, Rotary (wankel) engines, or any other mechanical device utilizing the combustion of a fuel to convert chemical energy to mechanical energy regardless of particular mechanical system employed.
Resistive sensors by their nature can be reduced in size to a much greater extent than voltaic sensors. In accordance with the invention, this characteristic is used to make a sub-miniature “micro-chip” oxygen sensor of particular usefulness in the small engine market. The invention also enables the possibility of individual cylinder control in multi-port fuel injection systems for large spark ignition engines such as automobiles. Another use is to provide a safety cut-off sensor to ensure engines are not running rich and creating noxious gases.